This invention relates to an improved crystalline semiconductor material and to an improved method for fabricating a highly pure, crystalline semiconductor having stoichiometric proportions. More particularly, this invention concerns itself with an improved method for producing highly pure, stoichiometric proportioned, indium phosphide crystalline materials especially useful as crystalline semiconductors or crystalline substrates for epitaxy procedures.
Indium phosphide, a group III-V semiconductor compound, has been found to be especially useful in two areas of technical interest. Its large band gap (1.35 eV) and high electron mobility make it useful as a semiconductor material, especially when employed in its highly pure form, and as a crystalline substrate for device fabrication by expitaxial deposition techniques. Thin film InP devices have great potential for use in integrated optic applications and high frequency microwave devices. However, it is extremely difficult to synthesize indium phosphide to the degree of purity necessary for its successful utilization as a thin film device.
A number of methods have been suggested for synthesizing indium phosphide. One method for growing fairly large crystals involves the direct reaction of elemental phosphorus with elemental indium. This method offers the advantage of producing a relatively pure product since there is no possibility of contamination by other reactants. Unfortunately, the reaction often leads to large pressure buildup with a resulting explosive potential. This method requires small, strongly sealed containers or bombs and is an expensive and dangerous method for conducting the reaction. Reaction methods involving compounds of indium and phosphorus, rather than elemental reactants, have also been suggested, but the resulting indium phosphide has often been lacking in the degree of purity needed to grow good single crystals.
Another method which has proven successful in producing highly pure product involves transporting phosphorus in the vapor phase from a source of solid red phosphorus in a first zone of a reaction vessel to a source of solid indium positioned in a second zone of the vessel which is thermally isolated from the first zone. This allows a reaction to occur at a controlled rate without overheating the phosphorus. Although this method provides a highly pure product, it nevertheless still suffers from an explosion problem in which explosions often occur during heating and cooling of the reaction vessel. A quartz ampoule is generally utilized as the reaction vessel and it was found that indium rich indium phosphide wetted and cracked the quartz. Then, the resulting high pressure of the phosphorus would cause the cracked quartz ampoule to explode.
In attempting to overcome this problem, it was found that if the phosphorus and indium were heated within narrow temperature ranges coupled with a specific programmed cooling cycle then explosions would not occur. The entire zone within the ampoule where the reaction between the solid indium and the phosphorus vapors occurs must be kept entirely within a narrow and limited temperature range of 1070.degree. to 1150.degree. C. while simultaneously maintaining the temperature at the red phosphorus interface at 546.degree. C. Maintaining these narrow temperature ranges and resulting 27.5 atmospheres of pressure, as well as the specific programmed cooling rate, not only produces a highly pure product of stoichiometric proportions, but eliminates the danger of explosion.